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C 25 and C 30 highly branched isoprenoid (HBI) alkenes are lipid biomarkers produced exclusively by certain diatoms and found in sediments worldwide. Here, for the first time, it is shown that the marine planktonic diatom Pseudosolenia calcar-avis (Schultze) (Sundström, B.G., 1986. The Marine Diatom Genus Rhizosolenia (A New Approach to the Taxonomy) (Ph.D. thesis). Lund University, Lund), isolated from near surface waters in Mecklenburg Bay (southwestern Baltic Sea) biosynthesizes one C 25:2 and two C 25:3 HBI alkenes previously reported in some benthic diatoms. The presence of the same C 25 HBI alkenes in surface sediments from Mecklenburg Bay and the Arkona Sea, as well as their co-occurrence with remnant frustules of P. calcar-avis in a sediment core from the northern Baltic Sea, indicates that these lipids are very likely specific biomarkers for this diatom in this region at least. Further, since P. calcar-avis has a tolerance for low salinities (8–10), we also suggest that the occurrence of some of the HBIs produced by this diatom might represent a useful proxy for measuring past changes in inflowing water from the North Sea. The same C 25 HBIs, previously found in sediments from the Caspian Sea and the Black Sea, may also be produced by P. calcar-avis in such settings. [ABSTRACT FROM AUTHOR]